When a home furnace lights and stops, the usual culprits are flame sensing, airflow limits, or gas supply issues.
If your heat starts, sputters, and quits, you’re seeing a common fault. The system tries to light, fails a safety check, and the board cuts gas. You can confirm the pattern and fix simple items in minutes. This guide shows steps and when to call a pro.
What That Start-And-Stop Pattern Tells You
A modern gas unit follows a script: call for heat, inducer, pressure switch, glow or spark, gas, flame proven, then blower. If flame drops in a few seconds, the sensor didn’t prove it. If it runs a minute or two then quits, the high-limit opened from low airflow. Short bursts on repeat point to lockout.
| Symptom You See | Most Likely Cause | Fast Check |
|---|---|---|
| Burners light, go out in 3–8 seconds | Dirty flame sensor or bad grounding | Clean the rod; check bracket is tight |
| Runs 1–5 minutes, then trips | High-limit opening from low airflow | Replace filter; open vents; check blower |
| No flame at all | No gas, failed igniter, or bad valve | Confirm gas shutoff is open; inspect igniter |
| Clicks, inducer runs, then stops | Pressure switch or venting issue | Look for blocked intake/exhaust |
Quick Safety Checks Before You Troubleshoot
Smell gas? Leave the space and contact your gas provider. Hear a loud bang at light-off? Stop and book a licensed tech. Test your CO alarm before heating season. The U.S. safety agency advises alarms on each level and near bedrooms. You can read the latest guidance in this CPSC winter heating notice. If the alarm chirps or shows a fault, change batteries or replace the unit before heat season.
Furnace Fails To Ignite Then Turns Off — Causes And Fixes
When burners light then drop, the control doesn’t see flame. The sensor is a small metal rod in the burner area. It sits in the flame and sends a tiny DC current back to the board. Soot, oxidation, or a loose mount weakens that current. The board treats that as a flame loss and closes the valve.
Clean The Flame Sensor The Right Way
Kill power at the switch or breaker. Pull the single screw holding the rod. Gently buff the rod with a fine abrasive pad or plain paper towel until the metal looks clean. Don’t sand it smooth like chrome; you just want the film gone. Wipe off dust. Reinstall snugly, keeping the ceramic insulator intact. Restore power and start a heat cycle. If the flame now holds, that was it.
When Cleaning Doesn’t Hold The Flame
If the flame still drops, inspect the ground path. The sensor bracket must have a clean metal-to-metal seat to the burner assembly. Tighten screws. Check the wire from the rod to the control. Any crack or brittle insulation is a red flag. If the rod looks pitted or the ceramic is cracked, replace the part. Sensors are low cost and easy to match by model.
Airflow Trips: Filters, Vents, And The High-Limit
A clogged filter or closed supply registers can bake the heat exchanger. The high-limit opens to protect the unit, shutting the flame. If heat runs a minute or more, and quits and restarts after a cool-down, suspect airflow.
Swap The Filter And Check The Vents
Slide out the filter and read the size. Install a fresh one with the arrow pointing toward the blower. A clean filter reduces strain and keeps the heat exchanger within range. Open room vents. Move furniture off returns. If the coil sits above the furnace in a combined system, shine a light at the underside; heavy lint needs a pro cleaning.
Blower Not Moving Enough Air
If the motor is slow to start, squeals, or cycles off, you may have a failing capacitor or motor. Many motors use a simple plug-in capacitor. If you are not trained, stop here. Book a tech to test amperage and spin. This protects the board and keeps wiring safe.
Ignition Problems: Hot Surface And Spark Systems
Hot surface igniters glow orange. If yours stays dark, the igniter could be cracked or the board isn’t sending power. Remove power, inspect the element, and look for hairline breaks. Spark systems tick and light the burners. If you hear clicking with no light, look for a ground issue or dirty burners.
Test Steps You Can Do
- Watch the ignition sequence with the door switch held in by tape.
- Listen for inducer, then look for glow or spark.
- If flame appears then drops fast, go back to the sensor steps.
Gas Supply And Venting Checks
Make sure the manual gas cock is inline with the pipe. If other gas appliances work but the furnace does not, you might have a local valve issue. High-efficiency units use PVC intake and exhaust; leaves or frost can block flow and trip the pressure switch. Clear the pipes outside.
When A Technician Is The Smart Move
Call a tech if you smell gas, see scorch marks, find melted wiring, or the unit trips the breaker. Also call if pressure switch codes keep returning, or if the heat exchanger shows rust or cracks. A pro can test combustion, draft, and CO and verify gas pressure.
| Part Or Service | Typical Cost Range | DIY Risk |
|---|---|---|
| Flame sensor | $20–$60 part; $120–$250 installed | Low when power is off |
| Hot surface igniter | $40–$120 part; $180–$350 installed | Medium; element is fragile |
| Blower capacitor | $10–$40 part; $120–$220 installed | Medium; discharge first |
| Inducer or pressure switch | $150–$600 installed | High; needs tests |
| Control board | $300–$800 installed | High; setup required |
Seasonal Care That Prevents Short Cycling
Set a filter reminder every 30–90 days. Rinse the condensate trap yearly. Clear leaves from intake and exhaust pipes each fall. Keep space around the unit open. During a tune-up, ask the tech to verify temperature rise.
A national program also encourages simple HVAC upkeep to boost safety and comfort. It recommends regular filter swaps and pro checks. Read the ENERGY STAR maintenance checklist to see the basic steps that keep a gas furnace steady and safe.
Step-By-Step Troubleshooting Walkthrough
1) Confirm Power And Call For Heat
Set the stat high. Turn on the service switch. Check the breaker. Close the blower door so the safety switch is pressed.
2) Watch The Startup Sequence
Inducer starts, then glow or spark. Next, the valve opens and burners light. The board wants to see flame in seconds.
3) If Flame Drops In Seconds
Shut off power. Pull and clean the sensor. Reinstall firmly. Confirm a solid ground. Try a heat call again. If the flame stays on, you solved it. If not, replace the sensor and retest.
4) If Heat Runs Then Trips
Swap the filter. Open vents and returns. Check the blower belt on old units. If it still trips, schedule a pro to measure static pressure and temperature rise.
5) If There’s No Flame
Check the gas cock and any shutoffs. Relight a standing pilot only if your model uses one and the label shows the steps. For hot surface or spark systems, inspect the igniter and wiring. Many boards post a code when the valve never opens; match it to the door chart.
6) Clear Soft Lockout After Repairs
Many boards pause after several failed tries. After you fix the cause, cut power for 30 seconds and restore it. Run a fresh cycle and watch the flame hold.
Why This Pattern Happens On Modern Units
Safety circuits guard the home and the appliance. The flame sensor proves burn, the high-limit keeps temps in range, and the pressure switch confirms draft. Any failed signal makes the board shut gas and pause. A tiny film on a sensor or a dusty filter can look like a big failure on day one of heating season.
When Replacement Makes Sense
If your unit is old or repairs keep stacking up, get quotes for a new setup. Match size to your home and ducts. Keep venting clear and filters fresh to avoid a repeat next fall.